Active heating and cooling of substrates in low-pressure processing environments involve transferring heat to and from the substrates through thermally conductive chucks. Operations performed on the substrates include physical vapor deposition (PVD), chemical vapor deposition (CVD), ion beam sputtering, annealing, and cleaning.
Many low-pressure substrate processing operations are best performed at controlled substrate temperatures. During operations involving thermal depositions (e.g., CVD and annealing), elevated temperatures accomplish much of the processing. Other operations, including some plasma-assisted operations (e.g., PVD), benefit from maintaining constant substrate temperatures or substrate temperatures that are adjusted throughout different stages of the operations.
Thermally conductive chucks support substrates, such as silicon wafers, for such low-pressure processing within vacuum processing chambers. The substrates are held in place upon the chuck by gravity or fixed more securely in place using mechanical or electrostatic clamps. Although some radiational heating or cooling can take place within the vacuum processing chambers, substrate temperatures are mainly regulated through an interface between the substrates and the thermally conductive chucks. An inert gas, such as argon, is circulated through or confined within the otherwise low-pressure interface to assist transfers of heat between the substrates and their supporting chucks.
Most such chucks are arranged for either heating or cooling the substrates. U.S. Pat. No. 4,680,061 to Lamont Jr. discloses examples of both. One of the Lamont Jr.""s chucks has a ceramic heating element mounted in a cavity of the chuck""s body next to a substrate. Another of Lamont Jr.""s chucks has coolant channels for withdrawing heat from a chuck body portion next to a substrate. Such one-way temperature controls lack the flexibility to regulate temperature fluctuations in opposite directions and slow processing times when different directions of heat transfer between substrates and chucks are needed.
Co-assigned U.S. Pat. No. 5,775,416 to Heimanson et al. discloses a temperature controlled chuck incorporating both heating and cooling units. Both units are embedded in the chuck bodyxe2x80x94the heating unit next to the substrate and the cooling unit next to the heating unit. An evacuatable cavity separates the heating and cooling units. Pressure in the cavity is controlled to regulate the rate of heat transfer from the heating unit to the cooling unit.
The Heimanson et al. chuck provides for more accurately controlling substrate temperatures and for shortening processing times. Predetermined substrate temperatures can be achieved, maintained, or changed in an orderly manner despite processing interactions, which can transfer thermal energy into the substrate. More rapid cool-downs following operations that take place at higher temperatures are also possible.
Despite the improved processing made possible by the Heimanson et al. chuck, cycling speeds between periods of thermal isolation and thermal communication between heating and cooling units are subject to the gas flow rates into and out of the cavity separating the two units. Substantial gas pressure is required to optimize heat transfers, and gas pressure approaching a vacuum is required to most effectively inhibit heat transfers.
This invention achieves more rapid cycling among or between substrate heating and cooling operations by providing a chuck with a relatively movable temperature conditioner. The chuck preferably has a pedestal in thermal communication with a substrate. The temperature conditioner, which can be a heating unit or preferably a cooling unit, is movable into and out of thermal contact with the pedestal. The movement of the temperature conditioner into and out of thermal contact with the pedestal provides for more rapidly changing substrate temperature, which improves processing control and shortens processing time.
One example of such a rapid thermal-cycle chuck includes a thermally conductive pedestal that supports a substrate for processing in the low-pressure processing environment. A temperature conditioner, which is located outside the low-pressure processing environment, is mounted for relative movement with respect to the thermally conductive pedestal. An actuator relatively moves the temperature conditioner with respect to the thermally conductive pedestal between a first position in enhanced thermal contact with the thermally conductive pedestal and a second position in reduced thermal contact with the thermally conductive pedestal. The two positions regulate rates of heat transfer between the temperature conditioner and the pedestal.
The temperature conditioner preferably includes a thermally conductive block having a substantial thermal mass along with a substantial interface area for contacting the pedestal, which at the second position can acquire a considerable temperature difference from the pedestal. When moved into thermal contact with the pedestal, the considerable temperature difference together with the substantial thermal mass and interface area supports a rapid transfer of heat between the temperature conditioner and the pedestal. Atmospheric pressure preferably prevails at the interface between the temperature conditioner and the pedestal to support the heat transfers.
A cooling unit within the temperature conditioner can provide active cooling of the substrate. Conduits circulate coolant through a thermally conductive block to withdraw heat, which can be discharged beyond the chuck. A heating unit within the pedestal can provide active heating of the substrate. One or more heating coils are embedded in a thermally conductive block of the pedestal. Rapid heating of the substrate preferably takes place with the heating unit activated and with the cooling unit of the temperature conditioner, which can also be activated, out of effective thermal contact with the pedestal. Rapid cooling of the substrate preferably takes place with the heating unit deactivated and with the cooling unit both activated and in effective thermal contact with the pedestal.
The actuator, which can take a variety of forms transforming fluid or electrical power into mechanical motion, preferably translates the temperature conditioner along an axis between the first and second positions. At the first position, the temperature conditioner is preferably in direct contact with the pedestal. At the second position, the temperature conditioner is preferably at a fixed distance from the pedestal sufficient to interrupt effective heat transfers between the temperature conditioner and the pedestal. A distance of no more than two millimeters is generally sufficient for this purpose. Fixed stops can be used to hold this distance.
A low-pressure processing system incorporating such a chuck includes an evacuatable chamber for processing a substrate within a low-pressure processing environment. The chuck has a pedestal that supports the substrate for processing within the low-pressure processing environment. A relatively movable temperature conditioner is located outside the low-pressure processing environment. An actuator relatively moves the temperature conditioner with respect to the pedestal for regulating heat transfers between the substrate and the temperature conditioner.
An interface between the temperature conditioner block and the pedestal is preferably exposed to pressure conditions (preferably atmospheric pressure) outside the evacuatable chamber. While in contact, gas molecules fill the interface and support efficient transfers of heat between the temperature conditioner block and the pedestalxe2x80x94effectively increasing the area of conductive contact between the two bodies. However, at a small separation between the temperature conditioner block and the pedestal of approximately one millimeter or more, the efficiency of heat transfer significantly decreasesxe2x80x94effectively isolating the two bodies.
The temperature conditioner is preferably a cooler block made of a thermally conductive material with an effective interface area with the pedestal sized for removing heat from the pedestal at a rate of at least 50 degrees centigrade per minute. Although choices of material and mass can provide some thermal capacity for absorbing transfers of heat, active cooling of the cooler block is preferred for extending the cooler block""s capacity to absorb heat. Circulation of coolant to and from the cooler block is preferred for actively removing heat from the block.
Cooling a substrate with such a low-pressure processing system involves locating the cooler block outside the low-pressure processing environment and relatively moving the cooler block into engagement with the pedestal supporting the substrate for processing within the low-pressure processing environment. The substrate together with the pedestal is cooled by transfers of heat to the cooler block. When sufficient heat has been transferred, the cooler block is relatively moved again to disengage from the pedestal. A minimal fixed separation between the disengaged cooler block and pedestal is preferably maintained to limit the amount of travel required to re-engage the cooler block and pedestal when more cooling is needed.
Cycles of heating and cooling of substrates can be carried out by locating both a heater and a cooler within the chuck of the low-pressure processing system. The heater is preferably located in direct thermal contact with the thermally conductive pedestal, and the cooler is preferably mounted outside the low-pressure processing environment for relative movement with respect to the pedestal. Activating the heater transfers heat to the pedestal for raising the substrate temperature. Relatively moving the cooler into engagement with the pedestal transfers heat from the pedestal for at least limiting the rise in substrate temperature. The heater can be deactivated upon engagement of the cooler for lowering the substrate temperature.
While the cooler is preferably relatively movable with respect to the pedestal, a heater could be similarly arranged for such relative motion to regulate the efficiency of heat transfers from the heater to the pedestal. In such a case, the cooler could be mounted in direct contact with the pedestal or remain a relatively movable component along with the heater.
Both the heater block and the cooler block can be divided into different zones to better control radial temperature gradients of the substrate. The different zones of the cooler block can be independently moved into contact with different portions of the pedestal, including the different zones of the heater.